I watched this on a whim several years ago, due to it becoming popular among my previous group of friends, and the appealing art style.
Normally, I HATE anime that is moe fanservice-based and has no pretty males, but K-on may be one of the few exceptions to this.
Well, the reason I hate such anime in the first place is because I am a feminist, so you can imagine that I hate seeing girls sexualized and used as nothing but pure fap-material(which many men do even for this seeminigly-innocent series).
The reason I was able to tolerate and even enjoy this anime(enough that I would spend $200 on a Tsumugi doll), was because I felt they made the girls somewhat realistic and NOT degraded for moe anime standards. Surprisingly, they weren't sexualized, and they were adorable without being over-the-top annoying like most cutesy anime girls are.
The story was like any typical daily-life anime I had seen before, except, I felt that it was very well-done for daily life anime standards. Something about how they paced it, or perhaps because of how they built up your excitement, it felt pretty good, despite how typical it is for a musical slice of life, when you stop to think about it.
Actually, this anime is what encouraged me to go and finally get piano lessons!
Really, the only thing I didn't like of the story was when Mio fell on the stage and accidentally revealed her underwear, as up-skirts are one of my least favorite things about ecchi, but it relieved me that they did not actually show it.
Also, there is no real conflict at all besides deadlines, grades, and getting Yui to learn the guitar(which she learned quickly). I personally think it would have added to the story if the girls at least had some sort of relationship conflict with each other, and maybe resolved it later on? Maybe if they had a rival at least?
It's either the art/animation or the music that makes this anime exceed my expectations. I don't quite know how to explain it in one word, other than "juicy"," squishy", or cute. I really quite love how they drew the characters' eyes and faces overall, and the movement in animation is really fluid, even little things like how their facial expressions change. They don't skim on their facial movements even when they're seen in chibi expressions. The art style for K-on is actually pretty unique, at least until some new anime series' with the same art-style came out afterwards. It will always be the "K-on style" to me, though.
Probably the best thing, along with the art/animation(possibly #1, though), is the music in this anime. Even to this day, years after the anime was at peak popularity, I still have some of their songs on my playlist. The music in this anime makes it a legit music anime that far exceeded my expectations that I had due to some of my friends saying "only creepy otakus like K-on".
Although, I am not too fond of the super cutesy soft-sounding songs, I absolutely love the rock-type songs like "Don't say Lazy" and "No thank you!!", as they are so catchy, energetic, and uplifting without being annoyingly cute(the lyrics definitely are cute/happy).
I should also add that Mio's strong voice in those two songs is quite empowering to hear in comparison to the soft pop songs.
As a person who generally hates cutesy anime girls... I felt that the characters were really charming. I would say that Yui and Mio were my favorite characters personality-wise, and Tsumugi my favorite design-wise(and because she plays the synth/piano).
I can't say I was too fond of Azusa, but she is definitely cute design-wise. I love her eyes. Other than her design, though, I felt like she was too boring of a character besides the whole cat ears thing.
One thing I particularly like about the character designs, is that they are not anorexic. They have realistic weights and heights typical of Japanese females in high school, and they get self-conscious about fat just like real girls(but not as annoyingly persistent).
This anime was pretty enjoyable for me. Normally, a non-comedy slice of life anime would have long periods of boringness, but this wasn't. It kept you excited through each episode, and something would always happen.
Watching this kind of felt like I was becoming friends with them myself. You kind of feel like you're a part of their world, and growing with them as new friends.
While watching this anime, this kept popping into my head: "Are you sure this is made for guys? It's too innocent and girly and friendship-y... It feels more like a cute shoujo anime, only without boys/romance!".
Overall, it's a good anime that is definitely above average compared with other slice-of-life, and I would recommend it to anyone who has free time and wants to watch a light-hearted anime.